Generally, shoes are used for the protection of wearers' feet. The shoes isolate and protect the wearers' feet from the environment when the wearers walk or run while wearing the shoes.
The average person takes about 6000 steps a day, which loads the feet of the walker with 432 tons, or over 400 tons, the approximate weight of an international jetliner.
The simplest way of developing a good shoe, which can absorb shocks applied to the sole of a wearer's foot, is to reduce the hardness of the shoe. When the term “cushion” is defined as a soft pad that reduces the maximum force, the term “good cushion” is defined as a soft pad that absorbs a sufficient amount of a shock so that the skeletal muscle system of the wearer is not damaged. However, besides cushioning, since actuating force is an important factor determining athletic performance, it is not necessarily desirable for a cushion system to absorb all of the force, in the interest of athletic performance. That is, this is a complicated problem, and it is difficult to define the optimum hardness of a shoe for absorbing a shock generated in reaction to skeletal muscle exertion. That is, information about the magnitude of the force is required in order to calculate the optimum hardness.
As the result of analysis of surface repulsive power using a pressure plate while running, the vertical magnitude of force between the surface of the ground and the foot of a runner has been determined to be two to three times the weight of the user. Considering that both feet of the runner contact the surface of ground, that a runner takes about 1000 steps per 100 meters, and that a magnitude of force as strong as two to three times the weight of the runner is repetitively applied to the runner's feet, if shocks are not sufficiently absorbed, runner's joints may degenerate, leading to lumbago. Accordingly, good running shoes that can absorb shocks are needed.
However, there is a trade-off between the shock absorbing function needed at the time of landing the foot on the ground and the repulsive elastic function needed at the time of separating the foot from the ground. If greater importance is placed on one of the two functions, the other function is somewhat compromised.
FIG. 1 shows the positions of the runner's foot and parts of the foot to which shocks are applied when the foot contacts the ground. FIG. 1(a) shows the position in which the back part of the sole of the runner's foot contacts the ground. In this case, the magnitude of a shock is greater than the runner's weight for the first 20 to 30 ms. In detail, the magnitude of a shock is equal to about 2.2 times the runner's weight. FIG. 2(b) shows the position in which the entire sole of the runner's foot contacts the ground after the back part contacts the ground. In this case, the upward power which supports the foot is greater than the runner's weight. FIG. 1(c) shows the position in which the front part of the sole of the runner's foot contacts the ground to prepare for the start of the next running step. The front part of the sole stays on the ground for about 100 ms and then leaves the ground. At this time, the front part of the sole is applied with a load equal to 2.8 times the runner's weight.
From the above, it can be seen that the total magnitude of shock applied to the sole of a runner or a walker is much greater than one would think. Accordingly, a variety of shock absorbing members, such as sponge foam, have been used in order to alleviate the magnitude of the shock. However, when such shock absorbing members are used, other problems arise. That is, although such shock absorbing members can absorb shocks somewhat, they also reduce the ground repulsive power, making running or walking difficult.
That is, in the structure of a shoe, the shock absorbing function, which must be strong when a wearer's foot lands on the ground, and the elastic repulsive power, which must be strong when the wearer's foot leaves the ground, are in a conflicting relationship. That is, if either one becomes stronger, the other one becomes weaker.